Forage frost protection potential of conifer silvopastures
Introduction
Agricultural production in temperate climate areas is constrained by cold weather which limits the growing season. Radiation frosts frequently injure vegetation early and late in the growing season even when air temperature is warm enough to maintain plant health. Radiation frost occurs at night when clear sky conditions result in a large net loss in long-wave radiation from plant surfaces and a lack of wind minimizes convective heat gain from air.
Obstructing the open sky from a plant’s upward field-of-view is one strategy for reducing the severity of radiation frost by providing a surface much warmer than open sky with which plants exchange long-wave radiation. Shade cloth mounted over vegetation is effective for reducing radiation frost, however, the cost limits its use to protection of high value products (Stamps, 1989, Igarashi et al., 1993, Teitel et al., 1996, Scowcroft et al., 2000).
Trees can provide an economical interface between crops and open sky that minimizes damage from radiation frost. Overstory trees are effectively used to prevent frost damage on coffee crops south of 20° latitude in South America (Caramori et al., 1996). In northern Europe, birch shelterwoods provide conifer saplings with some frost protection (Odin et al., 1984). While an analysis of tree overstory benefits on forage production has not been made, there have been observations of improved forage growth under conifer trees during cold weather (Sibbald et al., 1991, Brazoptos and Papanastasis, 1995).
The objective of this project is to quantify the relationship between tree canopy density in a conifer silvopasture and the severity of radiation frost.
Section snippets
Theoretical consideration
Net radiation, which is a major driver of plant climate, is most simply defined bywhere Ri (W m−2) is incoming radiation and Ro (W m−2) is outgoing radiation. During the daytime, both of these components contain short-wave and long-wave radiation. At night, however, the short-wave components are equal to zero.
The outgoing long-wave radiation from a surface is defined by the Stefan–Boltzmann equation aswhere σ is the Stefan–Boltzmann constant (5.6697×10−8 W m−2 K−4), ε the thermal
Materials and methods
The research site is a 0.7 ha area of 35-year-old, 17 m tall, mixed conifers on a farm site in southern West Virginia (37°46′W latitude 81°00′N longitude 860 m elevation). The site is dominated by white pine (Pinus strobus L.) and red spruce (Picea rubens Sarg.) with a few scattered pitch pine (Pinus rigida Mill.) and short-leaf pine (Pinus echinata Mill.). The trees are growing on a Gilpin soil (fine loamy, mixed, mesic Typic Hapludult). The understory vegetation is dominated by orchardgrass (
Results and discussion
The tree canopy cover ratios (mt), determined using the WinSCANOPY software, for the sky field-of-view at the eight silvopasture sensor sites were 0.77, 0.77, 0.75, 0.74, 0.70, 0.65, 0.60, and 0.55. These values represent the range of tree canopy cover within the conifer stand. The mt value for the adjacent field site without trees was 0.07 rather than 0 due to distant horizon obstruction.
The RFP sensors gave temperature values highly correlated with actual grass canopy temperature measured
Conclusions
A considerable amount of silvopastoral research has been focused on the impact of shade on forage accumulation and quality (Kephart and Buxton, 1993, Devkota et al., 1997, Lin et al., 1999). However, forages may be adversely impacted by radiation frosts early and late in the growing season. The ability of silvopastures to extend the growing season during these periods as a result of trees protecting forages from radiation cooling, to which open pastures are subjected, appears to be substantial.
Acknowledgements
I thank Barry L. Harter for the excellent technical assistance in carrying out this research and for design improvements suggested for the RFP sensor.
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